1. Field of the Invention
Exemplary embodiments of the present invention relate to a light detection device; and, particularly, to a light detection device that reduces current flow resistance due to an abrupt energy band gap change between a buffer layer and a light absorption layer, through the application of a multilayer band gap change layer having different energy band gaps.
2. Description of Related Art
Ultraviolet (UV) light has a shorter wavelength than the visible light, and was discovered for the first time by the German Chemist, J. W. Ritter, in 1801.
Here, UV radiation is defined as electromagnetic waves with a broad spectrum of wavelengths of about 397 to 10 nm, and the UV radiation with extremely short wavelength does not get distinguished easily with X-rays. Further, since the UV radiation is characterized by powerful chemical reactions, it is also called actinic rays, while infrared radiation is called heat rays.
The UV radiation with the wavelength of 400 nm or less is divided into several bands by wavelengths—UVA, UVB, and UVC. UVA measures 320 to 400 nm, corresponds to about 98% or more of sunlight that reaches the earth's surface, and negatively affects human skin, such as melanism or premature skin aging. UVB measures 280 to 320 nm, corresponds to about 2% of sunlight that reaches the earth's surface, and severely affects the human body, such as by causing skin cancers, cataracts, or erythema phenomenon. The UVB radiation is mostly absorbed by the ozone layer. However, due to destruction of the ozone layer, the amount of UVB radiation that reaches the earth's surface has been increased in many areas, and this causes severe environmental problems. UVC measures 200 to 280 nm, is all absorbed in the atmosphere, and thus, scarcely reaches the earth's surface. The UVC is mainly used for bacteriocidal action. The representative quantification of UV radiation influence exerted on the human body is the UV index that is defined as the incident level of UVB radiation.
A device that can sense the UV light may be a PMT (Photo Multiplier Tube) or a semiconductor device. Since the semiconductor device is cheaper and smaller than the PMT, it has widely been used. In the semiconductor device, GaN (Gallium Nitride) or SiC (Silicon Carbide) having an energy band gap that is suitable to UV sensing, is widely used.
Particularly, in the case of a device based on GaN, a Schottky junction type, MSM (Metal-Semiconductor-Metal) type, or PIN type device has been mainly used. In particular, the Schottky junction type device is generally preferred since its fabricating process is simple.
Here, in the Schottky junction type device, a buffer layer, a light absorption layer, and a Schottky junction layer are laminated in order on different substrates, a first electrode is formed on the buffer layer or the light absorption layer, and a second electrode is formed on the Schottky junction layer.
In this case, the buffer layer may be formed of a GaN layer, and the light absorption layer may be formed of an AlGaN layer. However, if the Al content of the AlGaN layer is equal to or higher than 15%, and the thickness thereof is equal to or larger than 0.1 μm, cracks may be produced due to differences in lattice mismatch and thermal expansion coefficient between the GaN layer and the AlGaN layer, thereby reducing yields.
In order to solve this, an AlN buffer layer may be used between the GaN buffer layer and the AlGaN light absorption layer. In the case of using the AlN buffer layer, a light detection reaction is reduced, due to the high energy band gap and insulating properties of the AlN layer.
Further, in the case of using the AlGaN layer as the light absorption layer, it is difficult to form an ohmic junction directly on the AlGaN layer, due to high contact resistance, if the Al content is equal to or higher than 15%. It is also not possible to obtain uniform Schottky junction characteristics due to high Schottky junction barrier.
In addition, if the thickness of the light absorption layer is set to 0.1 μm or less to prevent the cracks, the light absorption efficiency is deteriorated due to the thin thickness of the light absorption layer, and thus, the reaction thereof is also reduced.